Summary of Significant Accounting Policies, Earnings Per Average Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income
Basic net income applicable to common stock	$ 30,234	$ 36,633	$ 57,156	$ 62,557
Diluted net income applicable to common stock	$ 30,234	$ 36,633	$ 57,156	$ 62,557
Shares
Average number of basic common shares outstanding (in shares)	60,480,684	60,359,949	60,470,989	60,424,591
Effect of dilutive securities
Add: restricted stock (LITCP) (in shares)	320,122	266,186	305,746	253,435
Average number of diluted common shares outstanding (in shares)	60,800,806	60,626,135	60,776,735	60,678,026
Per Share Amount
Basic net income applicable to common stock (in dollars per share)	$ 0.50	$ 0.61	$ 0.95	$ 1.04
Diluted net income applicable to common stock (in dollars per share)	$ 0.50	$ 0.60	$ 0.94	$ 1.03